Dehydration is a state defined by tilting of the normal balance of water and/or electrolytes and salts towards depletion. Water in the body is found in complex with salts and ions. It is distributed in spaces around the cells; the extracellular compartment, and the fluid content within cells is termed as intracellular. The movement of water molecules through the cellular membranes also attracts ions and electrolytes with it. Sodium is the primary ion in the extracellular fluid while calcium is its corresponding anion. Potassium plays role in the intracellular compartment of water. 
The body can have depleted stores of water and ions; when the intake of water is not adequate to meet the body’s demands, or if there has been an excessive abnormal loss of water that cannot be compensated by mere intake of water. Normally, the human body can lose 1.5 liters per day of water and its associated salts and ions in the urine via the urinary tract. Water is also lost in feces via the gastrointestinal tract, and as sweating through the skin. The imperceptible evaporation of water from the skin and lungs comes under the heading of insensible water loss, which amounts to 0.9 liters per day. If this loss is not compensated by adequate intake of water, dehydration results. A person may lose about 50ml/kg of body fluids to more than 100ml/kg which presents as mild, moderate or severe dehydration.
1Types of Dehydration
Notwithstanding the common notion, dehydration is not only the loss of hydration of the body, but can also result from loss of electrolytes. Depending upon the proportion of loss of water, and electrolytes in different incidences, dehydration can be subdivided into three types 
- Isotonic dehydration: in cases where the loss of water and electrolytes, mainly sodium is in proportion, it is called isotonic dehydration. This is seen in cases of vomiting, diarrhea, excessive sweating and burns.
- Hypertonic dehydration: in situations where the loss of water is far greater than the loss of sodium, the disturbed balance of tonicity in the body is called hypertonic dehydration. This is commonly associated with diabetes insipidus, fever and high breathing rate.
- Hypotonic dehydration: this type accounts for the preferential loss of sodium and other ions compared to water.